Plasma can be created by temperatures over 2000°K. At this temperature
many high-velocity gas atoms collide with enough energy to knock
electrons off each other and thus become ionized. The material thus
created, shown as a glowing gas in Figure 10, does not behave
consistently as any of the three familiar states of matter: solid,
liquid, or gas. Plasma has been called a _fourth state of matter_. Since
we have difficulty in containing such high temperatures on earth, we
adopt the strategy of _seeding_. In this technique gases that are
ordinarily difficult to ionize, like helium, are made conducting by
adding a fraction of a percent of an alkali metal such as potassium.
Alkali metal atoms have loosely bound outer electrons and quickly become
ionized at temperatures well below 2000°K.
[Illustration: Figure 10 _Glowing plasma in experimental device at
General Atomic’s John Jay Hopkins Laboratory, San Diego. T-shaped
plasma gun provides data for research in thermonuclear fusion._
Courtesy Texas Atomic Energy Research Foundation.]
A helium-potassium mixture is a good enough conductor for use in an MHD
generator. In this plasma the electrons move rapidly under the influence
of the applied fields, though not as well as in metals. The positive
ions move in the opposite direction from the electrons, but the
electrons are much lighter and move thousands of times faster thus
carrying the bulk of the electrical current.